Mobile communication system and communication method for mobile communication system

ABSTRACT

A mobile communication system performing communication between a plurality of base stations and a mobile station is provided. A user data signal is transmitted with the transmission power controlled from one base station selected among the plurality of base stations. Control data signals are transmitted from the plurality of base stations to the mobile station, with the transmission power controlled in a similar way as the user data signal. The mobile station measures quality of the control data signals on a basis of each base station, and selects a base station based on the measured control data signal quality of each base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/826,962 filed on Apr. 16, 2004 now U.S. Pat. No. 7,133,682,which is a continuation of International PCT Application No.PCT/JP01/09173 filed on Oct. 18, 2001, pending.

FIELD OF THE INVENTION

The present invention relates to a mobile station and a base stationperforming wireless communication, and a mobile communication systemincluding a mobile station and a base station, and a communicationmethod for a mobile communication system.

BACKGROUND ARTS

As one of a communication system for the third generation wirelesscommunications (standardized by ITU-T as International MobileTelecommunications-2000, or IMT-2000), the W-CDMA (Wideband Code.Division Multiple Access) system has been adopted. In this W-CDMA,various techniques are introduced to improve communication quality(wireless transmission quality), which include soft handover (sitediversity), site selection diversity transmit power control (SSDT), andclosed-loop transmission power control.

The soft handover is a technique in which a mobile station issimultaneously connected with a plurality of base stations throughwireless links, and receives signals from the plurality of base stationsusing the RAKE receivers. At the time of the soft handover, the entireplurality of base stations performing the soft handover transmit signalson Dedicated Physical Channels (DPCH), and the mobile station receivesthe DPCH signals from the plurality of base stations.

The SSDT is a method for power control performed at the time of the softhandover, to solve a problem of increased interference on a downlink(i.e. a link directed to a mobile station from a base station) producedby the identical DPCH signals being transmitted to the mobile stationfrom the plurality of base stations at the time of the soft handover.The mobile station selects one of the base stations performing the softhandover as a primary cell, whereas other base stations are determinedas non-primary cells. Only the primary cell transmits signals on aDedicated Physical Data Channel (DPDCH) in the DPCH, and non-primarycells do not transmit any DPDCH signals. Among the DPCH signals,Dedicated Physical Control Channel (DPCCH) signals are transmitted forthe entire base stations performing the soft handover.

In the SSDT method, a mobile station measures the received signal codepower (RSCP) of a Common Pilot Channel (CPICH) transmitted with constantpower from each base station. The mobile station selects base stationsof which measurement result is higher than the predetermined thresholdas soft handover candidate. Among these soft handover candidates, themobile station selects the base station producing the maximum RSCP asprimary cell. By changing (updating) the primary cell at high speed, themobile station can receive the DPDCH with better reception quality.

The closed-loop transmission power control is a power control methodperformed in the following way: Both a mobile station and a base stationmeasure reception signal quality (i.e. transmission signal quality interms of the transmission side). Depending on the measurement result, atransmit power control (TPC) command is transmitted to a transmissionside so that a reception side can receive a signal with desired quality.The transmission side then controls the own transmission power based onthe TPC command. This control method aims to solve the far-to-nearproblem and reduce an effect caused by fading fluctuation. As ameasurement criterion for the receive signal quality, the SIR(signal-to-interference power ratio) is applied.

Here, in the conventional SSDT, particularly in an inner-loop control ofthe transmission power control on the downlink, a mobile stationmeasures the SIR of the DPCCH received from a primary cell, compares themeasurement value with a target SIR value, and generates a TPC commandin accordance with the comparison result. The mobile station thentransmits this TPC command to both the primary cell and the non-primarycells through the uplinks (links directed to the base stations from themobile station). According to the TPC command, the primary cell controlstransmission power of the DPDCH and the DPCCH on the downlink.Meanwhile, the non-primary cells control the transmission power of theDPCCH on the downlink, but do not control the transmission power of theDPDCH.

FIG. 6 shows a state of transmission power control on the downlink atthe time of the SSDT. A base station selector 102 provided in a mobilestation 100 measures the RSCP of the CPICH transmitted from n basestations 200 ₁-200 _(n) (where n is integer no less than 2), and selectsthe base station producing the maximum RSCP value as primary cell. Theselection result of the primary cell is transmitted to base stations 200₁-200 _(n) on uplink feedback information (FBI), in which identificationinformation of the base station indicating the primary cell is included.Thus, each base station 200 ₁-200 _(n) can identify whether the basestation of interest is the primary cell or the non-primary cell.

An SIR measurement section 103 provided in mobile station 100 measuresthe SIR of the DPCCH transmitted from the base station which has beenselected as primary cell by base station selector 102, among the DPCCHtransmitted from base stations 200 ₁-200 _(n). SIR measurement section103 then feeds the measurement result to a TPC bit generator 104. TPCbit generator 104 compares the measured SIR with a target SIR havingbeen set in advance, and generates a TPC command based on the comparisonresult. The generated TPC command is transmitted to base stations 200₁-200 _(n).

In base stations 200 ₁-200 _(n), an FBI bit extractor 204 extracts, fromthe received data, identification information of the base station havingbeen selected as primary cell. Based on this base station identificationinformation, a switcher (SW) 203 determines whether or not the basestation of interest is selected as primary cell. If the base station ofinterest has been selected as primary cell, the base station concernedoutputs a DPDCH data to a power controller 202, whereas if the basestation concerned is not selected as primary cell, the base stationconcerned does not output any DPDCH data to power controller 202.

Meanwhile, a TPC bit extractor 205 extracts the TPC command from thereceived data, and then feeds the TPC command to power controller 202.Power controller 202 controls the transmission power of the DPCCHaccording to the TPC command. Power controller 202 further controls thetransmission power of the DPDCH in case the DPDCH data is supplied fromswitcher (SW) 203 according to the TPC command. These power-controlledchannel data are transmitted to mobile station 100.

The power control by power controller 202 is performed using the samecontrol method, irrespectively of whether the base station concerned isthe primary cell or the non-primary cell, according to the same TPCcommand (that is, the same increase/decrease amount of power based onthe same increment/decrement). FIG. 7 shows such a conventional powercontrol method in a tabular form. Both the primary cell and thenon-primary cell increase the transmission power by 1 dB when the TPCcommand indicates ‘1’, or decrease the transmission power by 1 dB whenthe TPC command indicates ‘0’.

As such, in the conventional SSDT method, the primary cell selection isperformed independently of the transmission power control. Namely, theprimary cell selection is determined being referenced from the RSCP ofthe CPICH, whereas the transmission power control is performed by use ofthe TPC command determined by referencing the SIR of the DPCCH.Moreover, the TPC command is transmitted on each time slot bases, andtherefore the transmission power control is updated at each timeinterval T of the time slot (for example, T=0.667 ms). In contrast, theprimary cell selection information is transmitted using no less thanthree time slots, and therefore the primary cell is updated at timeintervals three times as long as the time interval T.

Now, according to the aforementioned method in which the primary cellselection and the transmission power control are performedindependently, there lies a problem that an optimal primary cellselection cannot always be guaranteed.

More specifically, although the base station transmitting the DPDCHsignals with better quality has to be selected, according to theconventional method, the criteria applied for the transmission powercontrol which effects the communication quality is different from thecriteria for the primary cell selection. As a result, there may be casesthat a base station which provides better communication quality be notselected as primary cell.

Also, because the period of updating the primary cell is longer than theperiod of updating the transmission power control, there may be casesthat updating the primary cell cannot follow the change of transmissionpower. As a result, a base station providing larger transmission powermay not be selected as primary cell.

Further, because the primary cell update period is long, there may alsobe cases that updating the primary cell cannot follow fadingfluctuations. This may also impede to receive the DPDCH signaltransmitted from a base station providing larger transmission power.

Also, conventionally, as having been illustrated in FIG. 7, both theprimary cell and the non-primary cells perform identical transmissionpower control based on the common TPC command, which may possibly makeit difficult to switch over from the primary cell to a non-primary cell.This may reduce effect of high-speed cell selection obtained by a rapidswitchover of primary cell to more optimal base station.

Moreover, because generally lower communication quality is provided by anon-primary cell than by the primary cell, the transmission error ratefor a TPC command may possibly be increased on the uplink also. In sucha case, there arises a problem that the base station may performtransmission power control based on an incorrect TPC command. As aresult, greater transmission power difference than transmission lossdifference may be produced between a plurality of base stations whichare soft handover candidates. This may produce increased interferenceamong the downlinks.

To cope with this problem, a method has been proposed in the TechnicalReport of IEICE, RCS 2000-164, published by the IEICE (the Institute ofElectronics, Information and Communication Engineers). According to theproposed method, which has been referred to as SIDTPC (site independentdiversity transmit power control), a mobile station measures the SIR ofthe signal from each base station after the RAKE receiver, and generatesthe TPC command so that each base station can perform independenttransmission power control.

However, according to this method having been proposed, it becomesnecessary to provide a large amount of bits in a TPC command to betransmitted in the uplink DPCCH, which becomes as many as the number ofbase stations, as compared with the conventional method. Or, in order tomake the number of the TPC command bits identical to the conventionalmethod, the transmission power control period becomes longer, which maydeteriorate capability to follow fading fluctuations.

DISCLOSURE OF THE INVENTION

Considering the above-mentioned background, it is a first object of thepresent invention to enable selection of a base station which transmitsa user data signal with better quality, in a communication conditionsuch that the user data signal is transmitted with power control fromone base station selected from among a plurality of base stations by amobile station.

It is a second object of the present invention to enable updating(switching) from the selected base station at high speed.

In order to attain the above-mentioned first object, according to afirst aspect of the present invention, a mobile station has radiochannels being set between the mobile station and a plurality of basestations, and performs communication with the plurality of basestations, in a communication condition such that one base stationselected among the plurality of base stations transmits a user datasignal which transmission power is controlled, and that the plurality ofbase stations including the selected base station transmit signalsincluding a control data signal which transmission power is controlledin a similar way as the user data signal. The mobile station includes: ameasurement section measuring, on a basis of each base station, qualityof the control data signal transmitted with the transmission powercontrolled; a selector selecting the base station transmitting the userdata signal, based on the quality of the control data signal from eachbase station measured in the measurement section; and a transmittertransmitting identification information for identifying the base stationselected by the selector, to the plurality of base stations.

In regard to the above-mentioned quality measurement,signal-to-interference power ratio is used in one embodiment of thepresent invention, or reception power is used in another embodiment.

According to the first aspect of the present invention, the transmissionof the user data signal from the base station with transmission powercontrolled is performed based on the quality of the control datatransmitted with the transmission power controlled in a similar way asthe user data signal. Therefore, as for the transmission of the userdata signal, it becomes possible to select the base station which cantransmit user data signal with the best quality, and accordingly, themobile station can receive this user data signal with the best quality.

Preferably, the mobile station further includes a generator which haspreset target quality and compares the target quality with the qualityof the base station selected by the selector among the quality setsmeasured by the measurement section, generating power controlinformation instructing to decrease the transmission power in case ofthe latter having better quality than the former, and instructing toincrease the transmission power in case of the former having betterquality than the latter.

In this way, the selection of the base station transmitting the userdata signal and the generation of the power control information areperformed based on the quality of the identical control data signal.Namely, the criterion for the base station selection and the criterionfor the power control information determination are commonized(unified).

Preferably, the mobile station further includes a generator generatingpower control information indicating how the transmission power of theplurality of base stations is to be controlled, based on the quality ofthe base station selected by the selector among the quality setsmeasured by the measurement section. The transmitter stores the powercontrol information generated by the generator, as well as theidentification information, into each time slot in a frame having aplurality of time slots, and transmits the power control information andthe identification information to the plurality of base stations.

In this way, the identification information of the base stationtransmitting the user data signal is transmitted in each time slot.Therefore, by use of each time slot data, it becomes possible to selectthe base station transmitting the user data signal. Thus, it becomespossible to select the base station (updating, or substitution, of thebase station) more rapidly than in the conventional method, and theaforementioned second object of the present invention can be achieved.As a result, it becomes possible to make the base station selectionfollow fading variations.

According to a second aspect of the present invention, a base stationhas radio channels being set between the base station and a mobilestation, and transmits a user data signal which transmission power iscontrolled to the mobile station only when the base station of interestis selected by the mobile station, and also transmits, to the mobilestation, signals including a control data signal which transmissionpower is controlled in a similar way as the user data signal,irrespective of whether or not the mobile station of interest isselected. The base station includes: a receiver receiving identificationinformation transmitted from the mobile station, representing the basestation which is selected by the mobile station based on the quality ofthe control data signal transmitted with the transmission powercontrolled; and a transmitter transmitting the user data signal to themobile station only when the identification information represents thebase station of interest.

According to the second aspect of the present invention, a communicationmethod for a base station is disclosed. The communication method isperformed in each plurality of base stations, in a communicationcondition such that radio channels are set between the plurality of basestations and a mobile station, that a user data signal is transmittedwhich transmission power is controlled to the mobile station from onebase station selected among the plurality of base stations, and thatcontrol data signals, which transmission power is controlled in asimilar way as the user data signal, are transmitted to the mobilestation from the plurality of base stations including the selected basestation. The communication method includes: receiving identificationinformation, transmitted from the mobile station, representing the basestation which is selected by the mobile station based on the quality ofthe control data signal with the transmission power controlled; and whenthe identification information represents the base station of interest,transmitting the user data signal with the transmission powercontrolled, and also transmitting the control data signal with thetransmission power controlled, whereas when the identificationinformation does not represent the base station of interest,transmitting the control data signal with the transmission powercontrolled, without transmitting the user data.

According to a second aspect of the present invention, the base stationtransmitting the user data signal is selected by the mobile station,based on the control data signal with the transmission power controlledsimilar to that performed against the user data signal. Thus, it becomespossible to select the base station capable of transmitting the userdata signal with the best quality, and the mobile station can receivethe user data with the best quality.

Preferably, each of the base stations further receives in the receiverthe power control information determined by the mobile station based onthe quality of the control data signal, indicating how the transmissionpower is to be controlled. The base station further includes a powercontroller which controls transmission power of both the user datasignal and the control data signal, based on the power controlinformation received by the receiver.

In this way, similarly to the first aspect, both the selection of thebase station transmitting the user data signal and the generation of thepower control information for the power control of the base station areperformed based on the same control data signal quality, and thus thecriterions are commonized (unified).

Further, preferably, the identification information of the base stationtransmitting the user data signal is received in each time slot of aframe having a plurality of timing sets. Thus, it becomes possible toselect the base station transmitting the user data signal in each timeslot. This enables faster base station selection (update of the basestation) than in the conventional method. As a result, updating the basestation becomes easier to follow fading fluctuations than in theconventional method.

In the second aspect of the present invention, according to a firstpreferred embodiment, when the identification information represents thebase station of interest, the power controller controls to increase thepower in case of the power control information instructing increase ofpower, and to decrease the power in case of the power controlinformation instructing decrease of power, whereas when theidentification information does not represent the base station ofinterest, the power controller controls to increase the power in case ofthe power control information instructing increase of power, andmaintains the present power in case of the power control informationinstructing decrease of power.

According to a second preferred embodiment, when the identificationinformation represents the base station of interest, the powercontroller controls to increase the power in case of the power controlinformation instructing increase of power, and to decrease the power incase of the power control information instructing decrease of power,whereas when the identification information does not represent the basestation of interest, in case of the power control informationinstructing increase of power, the power controller controls to increasethe power with a smaller increment than the increment of when theidentification information represents the base station of interest, andin case of the power control information instructing decrease of power,the power controller controls to decrease the power with a smallerdecrement than the decrement of when the identification informationrepresents the base station of interest.

According to a third preferred embodiment, when the identificationinformation represents the base station of interest, the powercontroller controls to increase the power in case of the power controlinformation instructing increase of power, and to decrease the power incase of the power control information instructing decrease of power,whereas when the identification information does not represent the basestation of interest, the power controller controls to maintain thepresent power in case of the power control information instructingeither increase or decrease of power.

As such, according to these preferred embodiments, it is set to performdifferent power control between in the base station selected by themobile station and in the base station not selected. Therefore, itbecomes unnecessary for the mobile station to generate and transmitindividual power control information for each base station. This enablesreduction of power control information field in each time slot.

Also, with regard to the transmission power control method based on thepower control information, the method employed for the base stationselected by the mobile station differs from the method for other basestations not selected. Accordingly, the transmission power controlresults in higher probability of any base station not selected becomingsubstituted for the base station having been selected. In other words,updating the base station selection is promoted. Moreover, even whenincorrect power control information is transmitted due to an uplinktransmission failure, the transmission power difference between the basestation selected and the base stations not selected can be preventedfrom extending more than the degree of transmission loss difference. Asa result, increase of the interference on the downlink can be avoided.

According to a third aspect, in a mobile communication system having aplurality of base stations and a mobile station, with radio channelsbeing set between the plurality of base stations and the mobile station,in which a user data signal is transmitted with transmission powercontrolled to the mobile station from one base station selected amongthe plurality of base stations, and control data signals, whichtransmission power is controlled in a similar way as the user datasignal, are transmitted to the mobile station from the plurality of basestations including the selected base station, the mobile stationincludes: a measurement section measuring quality of each control datasignal on a basis of each base station; a selector selecting the basestation transmitting the user data signal, based on the quality of thecontrol data signal from each base station measured in the measurementsection; and a transmitter transmitting identification information foridentifying the base station selected by the selector to the pluralityof base stations. Each plurality of base stations includes a transmittertransmitting the user data signal to the mobile station only when theidentification information transmitted from the mobile stationrepresents the base station of interest.

According to the third aspect, in a communication method for a basestation among a plurality of base stations, with radio channels beingset between the plurality of base stations and a mobile station,performed in a communication condition such that a user data signalwhich transmission power is controlled is transmitted to the mobilestation from one base station selected among the plurality of basestations, and that control data signals, which transmission power iscontrolled in a similar way as the user data signal, are transmitted tothe mobile station from the plurality of base stations including theselected base station, mobile station: measures quality of the controldata signals on a basis of each base station; selects the base stationtransmitting the user data signal, based on the measured quality of eachcontrol data signal transmitted from each base station; and transmitsidentification information for identifying the selected base station, tothe plurality of base stations, and each plurality of base stations:when the identification information transmitted from the mobile stationrepresents the base station of interest, transmits the user data signalwhich transmission power is controlled based on the power controlinformation to the mobile station, and also transmits the control datasignal with the transmission power controlled, whereas when theidentification information does not represent the base station ofinterest, transmits the control data signal with the transmission powercontrolled, without transmitting the user data.

According to this third aspect, it is possible to obtain the samefunctions and effects as in the first and second aspects of the presentinvention.

According to another aspect, a mobile station performing communicationwith a plurality of base stations includes: a measurement sectionmeasuring, on a basis of each base station, quality of control datasignals which are transmitted from the plurality of base stationsaddressed to the mobile station of interest and which transmission poweris controlled; a selector selecting a base station transmitting a userdata signal addressed to the mobile station of interest, based on thequality of the control data signals which are addressed to the mobilestation of interest and measured in the measurement section; and atransmitter transmitting identification information for identifying thebase station selected by the selector, to the plurality of basestations.

Further, according to still another aspect of the present invention, abase station performing communication with a mobile station includes: areceiver receiving identification information transmitted from themobile station, representing the base station which is selected based onquality of control data signals which transmission power is controlled;and a transmitter when the identification information represents thebase station of interest, transmitting with the transmission powercontrolled, both a user data signal addressed to the mobile station andthe control data signal addressed to the mobile station, whereas whenthe identification information does not represent the base station ofinterest, transmitting the control data signal addressed to the mobilestation with the transmission power controlled, without transmitting anyuser data addressed to the mobile station of interest.

Still further, according to another aspect of the present invention, ina mobile communication system performing communication between aplurality of base stations and a mobile station, the mobile station:measures quality of control data signals, which are transmitted with thetransmission power controlled from the plurality of base stations andaddressed to the mobile station of interest, on a basis of each basestation; selects the base station transmitting: a user data signaladdressed, to the mobile station of interest, based on the measuredquality of the control data signals which are transmitted from theplurality of base stations and addressed to the mobile station ofinterest; and transmits identification information for identifying theselected base station, to the plurality of base stations. The basestation: when the identification information transmitted from the mobilestation represents the base station of interest, transmits with thetransmission power controlled, both the user data signal addressed tothe mobile station and the control data signal addressed to the mobilestation of interest, whereas when the identification information doesnot represent the base station of interest, transmits the control datasignal addressed to the mobile station of interest with the transmissionpower controlled, without transmitting any user data addressed to themobile station of interest.

Further scopes and features of the present invention will become moreapparent by the following description of the embodiments with theaccompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating a portion of the mobilecommunication system according to an embodiment of the presentinvention.

FIG. 2A shows a frame format of an uplink DPCH (DPDCH and DPCCH).

FIG. 2B shows another frame format of an uplink DPCCH and DPCCH).

FIG. 3 shows a frame format of a downlink DPCH.

FIG. 4A shows a power control method according to an embodiment of thepresent invention.

FIG. 4B shows another power control method illustrated in a tabularform.

FIG. 4C shows still another power control method illustrated in atabular form.

FIG. 5 shows a block diagram illustrating a portion of a mobilecommunication system according to another embodiment of the presentinvention.

FIG. 6 shows an aspect of transmission power control for the downlink incase of the conventional SSDT.

FIG. 7 shows the conventional power control method illustrated in atabular form.

THE PREFERRED EMBODIMENTS OF THE INVENTION

The preferred embodiment of the present invention is describedhereinafter referring to the charts and drawings. However, it is notedthat the following description merely exemplifies the embodiments of theinvention, and accordingly the scope of the invention is not limited tothe embodiments described below.

FIG. 1 shows a block diagram illustrating a portion of the mobilecommunication system according to one embodiment of the presentinvention. This mobile communication system, as an example, employsW-CDMA (Wideband-Code Division Multiple Access), the communicationstandard of the third generation wireless communications (IMT-2000). Themobile communication system is provided with a mobile station 1, n basestations (where, n is an integer no less than 2) 2 ₁-2 _(n), and a radionetwork control unit (base station control unit) 3.

Mobile station 1 is exemplified by portable telephone, automobiletelephone, and personal digital assistance, which perform radiocommunication with the entire base stations 2 ₁-2 _(n), or a portionthereof, using the Code Division Multiple Access (CDMA). This mobilestation 1 is provided with a receiver 11, m SIR measurement sections 12₁-12 _(m) (where, m is an integer no less than 2), a base stationselector 13, a switcher (SW) 14, a TPC bit generator 15, a transmitter16, and antennas 17, 18.

Antenna 17 receives downlink signals (data) transmitted from basestations 2 ₁-2 _(n) (hereafter each base station 2 ₁-2 _(n) isgenerically referred to as base station 2, except for the casesparticularly necessary to distinguish) using 2 GHz-band radio waves.Antenna 17 then supplies the received data to receiver 11. Antenna 18transmits uplink data supplied from transmitter 16 to base station 2using the 2 GHz-band radio waves. Here, these antennas 17, 18 mayotherwise be combined into a single antenna for both transmission andreception use.

The physical channels for the signals to be transmitted on the downwarddirection (downlink) include Dedicated Physical Channel (DPCH), CommonPilot Channel (CPICH), etc. The physical channels for the signals on theupward direction (uplink) include Dedicated Physical Channel (DPCH),Physical Random Access Channel (PRACH), etc.

Each channel includes frames (radio frames) each having a plurality oftime slots, and communication data are conveyed in these time slots.Each frame has a length of, for example, 10 ms, in which 15 timeslotsare included.

Both the uplink DPCH and the downlink DPCH are individually assigned foreach mobile station, and have Dedicated Physical Data Channel (DPDCH)and Dedicated Physical Control Channel (DPCCH).

One or more DPDCH are assigned for mobile station 1 which uses the DPCH,for use in conveying user data (voice data, character data, image data,etc.) of a user who uses mobile station 1. Meanwhile, one DPCCH isassigned for mobile station 1 which uses the DPCH, for use in conveyingphysical-layer control data in the DPCH. In the downlink, the DPDCH andthe DPCCH are time-multiplexed within one time slot, whereas in theuplink, the DPDCH and the DPCCH are I/Q-multiplexed on a frame-by-framebasis.

At the time of the SSDT (site selection diversity transmit powercontrol), the DPDCH is set only between mobile station 1 and basestation 2 having been selected as primary cell (hereafter referred to asprimary cell 2), whereas the DPDCH is not set between mobile station 1and base station 2 determined as non-primary cell (hereafter referred toas non-primary cell 2).

FIG. 2A shows the frame structure of the uplink DPCH (DPDCH and DPCCH).As described earlier, each radio frame has a plurality of time slots (15time slots, TS). Each time slot includes I/Q-multiplexed data of theDPDCH and the DPCCH.

The DPDCH includes user data, as described earlier. The DPCCH includes apilot bit of a known pattern used for channel estimation in the pilotsymbol aided coherent detection, a transport format combinationindicator (TFCI), a transmit power control (TPC) bit (or TPC command) ofbase station 2, and feedback information (FBI).

One bit is assigned for the TPC bit. In this TPC bit, ‘1’ is set whenthe transmission power of base station 2 is to be increased, while ‘0’is set when the transmission power of base station 2 is to be decreased.

Further, according to the embodiment, fast cell selection (FCS) bits (orFCS command) are included in the uplink DPCCH. These FCS bits may beincluded in the FBI field shown in FIG. 2A, or accommodated in an FCSfield, as shown in FIG. 2B, when the FCS field is provided separatelyfrom the FBI field in the time slot (for example, as in the case of theextended 3GPP specification, the 3rd Generation Partnership Projects.)

The FCS bits represent identification information indicating primarycell 2 selected at the time of the SSDT. As this identificationinformation, for example, a temporary cell ID assigned for each basestation 2 at the time of the SSDT is used. Namely, the base stationhaving the temporary cell ID represented by the FCS bits is the primarycell, and other base stations are the non-primary cells.

FIG. 3 shows the frame structure of the downlink DPCH. Similarly to theuplink DPCH, the downlink DPCH radio frame includes a plurality of timeslots (15 time slots, TS).

In each time slot of the downlink DPCH, the DPDCH and the DPCCH aretime-multiplexed, as described before. The DPCCH includes TPC bit, TFCI,and pilot bit for mobile station 1.

Now, receiver 11 performs amplification, de-spreading, RAKE combining,frequency conversion, A/D conversion, etc. against the signals receivedby antenna 17. Receiver 11 supplies the DPDCH data included in thereception signal to a data processor, and supplies the DPCCH data to SIRmeasurement sections 12 ₁-12 _(m).

Data processor performs predetermined process against the DPDCH data,and thereafter outputs voice data included in the data to a speaker etc.(not shown), and displays character data, image data, etc. included inthe data onto a display unit (not shown) such as a liquid crystaldisplay.

The reception signal of the DPCCH is Rake-combined by receiver 11 on abasis of each base station 2, and thereafter demultiplexed into theDPCCH data of each base station 2. The demultiplexed DPCCH data of eachbase station 2 is supplied to each SIR measurement sections 12 ₁-12_(m). For example, the DPCCH of base station 2 ₁ is supplied to SIRmeasurement section 12 ₁, the DPCCH of base station 2 ₂ is supplied toSIR measurement section 12 ₂, and so on.

The above value m is set equal to the number of base stations 2 to beselected as candidate for soft handover, i.e. the number of the activesets. Therefore, in ordinary cases, m is set smaller than n (m<n).Further, even on receipt of signals from more than m base stations 2,receiver 11 selects the radio waves received from m base stations 2.Thereafter receiver 11 performs RAKE combining against the signalsreceived from each base station 2, and supplies the Rake-combinedsignals to SIR measurement sections 12 ₁-12 _(m).

Here, the active sets are updated periodically, as mobile station 1moves. Also, when one cell is divided into a plurality of sectors, eachdivided sector is assigned as active set.

SIR measurement sections 12 ₁-12 _(m) (hereafter generically referred toas SIR measurement section 12, except for the cases when discriminationis required in particular) measure an SIR (signal-to-interference powerratio) of the received DPCCH data corresponding to each base station 2on a time slot basis, and supplies the SIR measurement value for eachtime slot to base station selector 13 and switcher 14.

Base station selector 13 determines the maximum value of the SIR bycomparing m SIR data supplied from SIR measurement section 12. Basestation selector 13 then selects base station 2 corresponding to themaximum SIR value as primary cell. Thereafter, base station selector 13supplies identification information representing the selected primarycell (for example, temporary cell ID) as the FCS bits to switcher 14 andtransmitter 16.

Here, the identification information assigned to each base station 2,for example the temporary ID, is notified in advance both to basestation 2 and mobile station 1 from radio network control unit 3. Inaddition, when there are two or more base stations 2 producing theidentical maximum SIR value, any arbitrary one of base stations 2 isselected as primary cell. Base stations 2 other than primary cell 2become non-primary cells.

According to the FCS bits supplied by base station selector 13, switcher14 sets the own switch condition so that the SIR of primary cell 2indicated by the FCS bits is output to TPC bit generator 15. Thus, themaximum SIR value is supplied to TPC bit generator 15 via switcher 14.

In TPC bit generator 15, a target SIR determined by non-illustratedouter-loop transmission power control is set in advance. TPC bitgenerator 15 compares the maximum SIR value supplied from switcher 14with the preset target SIR. When the maximum SIR value is greater thanthe target SIR, TPC bit generator 15 outputs to transmitter 16 the TPCbit ‘0’ (DOWN bit), which is an instruction to decrease the transmissionpower. When the maximum SIR value is smaller than the target SIR, TPCbit generator 15 outputs to transmitter 16 the TPC bit ‘1’ (UP bit),which is an instruction to increase the transmission power. When themaximum SIR equals to the target SIR, whichever instruction ofincreasing or decreasing the transmission power is applicable. One ofthe instructions is set in advance in TPC bit generator 15.

The DPDCH data supplied from the data processor, as well as data relatedto other channels (not shown), is input to transmitter 16, in additionto the FCS bits supplied from base station selector 13 and the TPC bitsupplied from TPC bit generator 15. The DPDCH data includes voice datainput by the user from a microphone (not shown), image data input froman imaging device such as a CCD camera, and the like.

Transmitter 16 performs processing against these input data, such asfrequency conversion, spreading, D/A conversion, orthogonal modulation,and amplification. Transmitter 16 then transmits the processed data tobase station 2 via antenna 18.

The TPC bit and the FCS bits are transmitted on the DPCCH as shown inFIGS. 2A, 2B explained earlier.

The TPC bit consisting of one bit is included in each time slot in oneframe (for example, each time slot in 15 time slots). Using this TPC bittransmitted in each time slot, base station 2 can perform transmissionpower control, as will be described later. For example, in case that oneframe length (in terms of time) is 10 ms, and one frame includes 15 timeslots (i.e. 1 time slot=0.667 ms), the transmission power control isperformed at the rate of 1,500 times per second.

Also, as for the FCS bits, the entire bits are included in each timeslot in one frame. Namely, in one time slot, the FCS bits necessary foridentifying each of m base stations 2 of active sets are included. Forexample, when m=8, the FCS bits of at least 3 bits are included in onetime slot. Thus, using the FCS bits included in one time slot, each basestation 2 can determine whether the base station concerned is selectedas primary cell. As a result, the primary cell is-updated at the sameintervals as the power control is performed using the TPC bit (forexample 1,500 times per second). Thus, high-speed cell selection isachieved.

Now, each base station 2 ₁-2 _(n) has the identical configuration,including transmitter 21, power controller 22, switcher (SW) 23, FCS bitextractor 24 TPC bit extractor 25, receiver 26, and antennas 27, 28.

Antenna 28 receives uplink signals (data) transmitted from mobilestation 1, using the 2 GHZ-band radio waves, and supplies the receiveddata to a receiver 26. Antenna 27 transmits downlink signals (data)supplied from transmitter 21 to mobile station 1, also using the2-GHz-band radio waves. These antennas 27, 28 may otherwise be combinedinto a single antenna for both transmission and reception use.

Receiver 26 performs the same processing against the reception data asreceiver 1 in mobile station 1 described earlier. Receiver 26 thensupplies the received DPDCH data to radio network control unit 3, andalso supplies the received DPCCH data to TPC bit extractor 25 and FCSbit extractor 24.

Radio network control unit 3 transmits the received DPDCH data to thebase station 2 concerned, or another base station 2, or anon-illustrated core network. This reception data is finally transmittedto another mobile station, or a server, terminal, etc. in anothercommunication network (for example, the Internet).

TPC bit extractor 25 extracts the TPC bit from each time slot of theDPCCH, and supplies the extracted TPC bit to power controller 22. FCSbit extractor 24 extracts the FCS bits from each time slot of the DPCCH,and supplies the extracted FCS bits to switcher 23 and power controller22.

The transmission data of the DPDCH supplied from radio network controlunit 3 is input to switcher 23, in addition to the FCS bits suppliedfrom FCS bit extractor 24.

Switcher 23 determines whether the FCS bits supplied from FCS bitextractor 24 are identification information indicative of the basestation concerned. Switcher 23 sets the own setting conditions dependingon the above determination result as described in the following: If theidentification information represented by the FCS bits specify the basestation concerned (in other words, if the base station concerned hasbeen selected as primary cell), switcher 23 supplies the DPDCHtransmission data to power controller 22. On the other hand, if theidentification information represented by the FCS bits does not specifythe base station concerned (in other words, if the base stationconcerned is a non-primary cell), switcher 23 does not supply the DPDCHtransmission data to power controller 22. With these settings, the SSDTis performed, and the DPDCH transmission data is transmitted from theprimary cell only, and not transmitted from any non-primary cells.

Because the FCS bits are supplied to switcher 23 at each time slot ofthe DPCCH, the above-mentioned determination whether the base stationconcerned is the primary cell, and the setting whether the DPDCHtransmission data is to be supplied to power controller 22 are performedat the intervals of the time slots (for example, 0.667 ms or 1,500 timesper second) In such a way, updating of the primary cell is performed atthe time slot intervals.

When there are a plurality of mobile stations which are communicatingwith one base station 2, the DPDCH transmission data with regard to theplurality of mobile stations are input to switcher 23. At the same time,the FCS bits and the TPC bit in the DPCCH are transmitted from theplurality of mobile stations. In this case, switcher 23 sets whether ornot the DPDCH transmission data for each mobile station is to besupplied to power controller 22, based on the FCS bits from each mobilestation. For example, when a mobile station A selects base station 2 ₁as primary cell, whereas a mobile station B does not select base station2 ₁ as primary cell, switcher 23 in base station 2 ₁ supplies the DPDCHtransmission data for the mobile station A to power controller 22,whereas switcher 23 does not supply the DPDCH transmission data for thebase station B to power controller 22.

The DPCCH transmission data is also input to power controller 22 of basestation 2 from radio network control unit 3, in addition to the TPC bitsupplied from TPC bit extractor 25, the FCS bits supplied from FCS bitextractor 24, and the DPDCH transmission data supplied from switcher 23when the base station concerned is selected as primary cell.

Based on the FCS bits and the TPC bit input, power controller 22controls the transmission power of both the DPCCH transmission data andthe DPDCH transmission data supplied from switcher 23, to increase ordecrease the transmission power.

FIG. 4A shows a power control method according to the embodiment of thepresent invention in a tabular form. Power controller 22 in base station2 determines whether the base station concerned has been selected asprimary cell, using the FCS bits. Also, power controller 22 determineswhether the TPC bit is ‘1’ (UP bit), or ‘0’ (DOWN bit).

When the TPC bit is ‘1’, power controller 22 in primary cell 2 increasesthe transmission power of both the DPDCH and the DPCCH by 1 dB, whereaswhen the TPC bit is ‘0’, power controller 22 decreases the transmissionpower of both the DPDCH and the DPCCH by 1 dB.

On the other hand, in non-primary cell 2, switcher 23 inhibits to inputthe DPDCH data to power controller 22. Accordingly, power controller 22in base station 2 of non-primary cell does not perform the transmissionpower control for the DPDCH (power control is set Off). Meanwhile, powercontroller 22 in non-primary cell 2 performs transmission power controlfor the DPCCH in the following manner. When the TPC bit is ‘1’, powercontroller 22 increases the DPCCH transmission power by 1 dB. When theTPC bit is ‘0’, power controller 22 neither increases nor decreases theDPCCH transmission power (that is, the increase/decrease amount is ±0dB).

Additionally, as for the transmission power, an upper limit is set inadvance in power controller 22. When the power value is to exceed theupper limit, power controller 22 does not increase the power, despitethe TPC bit ‘1’.

The transmission data of the DPCCH and the DPDCH, to which theabove-mentioned power control has been performed, are supplied totransmitter 21.

To transmitter 21, in addition to the DPCCH data (and the DPDCH data),data related to other channels (not shown) are supplied. Transmitter 21performs the same processing as for mobile station 1 described earlieragainst these channel data, and transmits these channel data via antenna27.

As such, inner-loop power control is performed in a closed loop formedof receiver 11, SIR measurement section 12, switcher 14, TPC bitgenerator 15, and transmitter 16 in mobile station 1, and also in aclosed loop formed of receiver 26, TPC bit extractor 25, powercontroller 22 and transmitter 21 in base station 2.

According to the embodiment of the present invention, transmission powercontrol is performed in a different manner between in the primary celland in the non-primary cell, against the common TPC bit. Accordingly, itis not necessary to generate and transmit different TPC bits torespective base stations, and thus the TPC bit field can be reduced insize.

Further, in non-primary cell 2, the transmission power is not decreasedwhen the TPC bit is ‘0’, whereas the transmission power, is increasedwhen the TPC bit is ‘1’. This increases the probability of the SIR ofnon-primary cell 2 becoming greater than the SIR of primary cell 2.Accordingly, the probability of non-primary cell 2 being selected asprimary cell 2 becomes greater In other words, easier substitution ofprimary cell 2 is promoted. As a result, mobile station 1 can receive aDPDCH data transmitted from base station 2 with better communicationquality.

Further, even when an incorrect TPC bit is transmitted due to atransmission failure in the uplink, the transmission power differencebetween the primary cell and the non-primary cell can be prevented fromextending more than the degree of transmission loss difference. As aresult, increase of the interference on the downlink can be avoided. Forexample, even when an incorrect TPC bit of ‘0’ is transmitted to thenon-primary cell due to a transmission failure, despite the correctvalue ‘1’, the transmission power of the non-primary cell is notdecreased. Accordingly, it becomes possible to avoid an excessivelylarge amount of variation in the transmission power difference producedbetween the primary cell and the non-primary cell.

Moreover, according to the embodiment of the present invention, theselection of primary cell 2 is performed based on the SIR, which is oneof reception characteristics of the DPCCH on which the power control isperformed. A base station 2 having the maximum SIR value is selected asprimary cell. There is a correlation between whether the SIR of theDPCCH is large (that is, whether the communication quality is good) andwhether the SIR of the DPDCH, in which the similar power control isperformed, is large. Therefore, primary cell 2 having been selectedbased on the SIR of the DPCCH is to provide good communication qualityof the DPDCH. Thus, on the occurrence of the SSDT, it becomes possiblefor mobile station 1 to receive the DPDCH from the base station whichprovides good communication quality.

Further, according to the embodiment, selection of the primary cell 2 isperformed at the time slot intervals. Accordingly, updating of primarycell 2 is performed more rapidly than that performed in the conventionalmethod. It becomes possible to follow fading variations at higher speedthan before. Even when the fading varies abruptly, updating the primarycell at high speed makes it possible to avoid a bad influence to thecommunication quality.

FIG. 4B shows another power control method illustrated in a tabularform. The transmission power control in base station 2 acting asnon-primary cell is different from the power control method shown inFIG. 4A, though the transmission power control in base station 2selected as primary cell is identical. Namely, the DPCCH transmissionpower in non-primary cell 2 is increased by 0.5 dB when the TPC bit is‘1’, and is decreased by 0.5 dB when the TPC bit is ‘0’.

In this method, the power is decreased to a greater extent in theprimary cell than in the non-primary cell, in case of decreasing thetransmission power. This can also promote easier substitution of theprimary cell. Moreover, even when an incorrect TPC bit is transmitteddue to an uplink transmission failure, the transmission power differencebetween the primary cell and the non-primary cell can be prevented fromextending more than the degree of transmission loss difference. As aresult, an interference increase on the downlink can be avoided.

FIG. 4C shows still another power control method illustrated in atabular form. According to this method, when base station 2 is anon-primary cell, the DPCCH transmission power is controlled to beinvariable, irrespective of the TPC bit of ‘1’ or ‘0’.

With this method also, the power is decreased to a greater extent in theprimary cell than in the non-primary cell, in case of decreasing thetransmission power. This can also promote easier substitution of theprimary cell. Moreover, even when an incorrect TPC bit is transmitteddue to an uplink transmission failure, the transmission power differencebetween the primary cell and the non-primary cell can be prevented fromextending more than the degree of transmission loss difference. As aresult, an interference increase on the downlink can be avoided.

As another embodiment of the present invention, mobile station 1 selectsa primary cell by comparing the DPCCH power, other than the SIR of theDPCCH. FIG. 5 shows a block diagram illustrating a portion of the mobilecommunication system according to this alternative embodiment of thepresent invention.

In the mobile communication system shown in FIG. 5, identical symbolsare assigned to configuration elements identical to those shown in FIG.1, and detailed description of these configuration elements is omitted.In the mobile communication system shown in FIG. 5, differently fromthat shown in FIG. 1, the SIR measurement sections 12 ₁-12 _(m) inmobile station 1 in FIG. 1 is replaced by power measurement sections 19₁-19 _(m) in FIG. 5, so as to measure the DPCCH reception power (RSCP:received signal code power) received in receiver 11.

With this configuration, a base station selector 20 determines themaximum power value among m power values measured in power measurementsections 19 ₁-19 _(m), and outputs as the FCS bits the identificationinformation of base station 2 corresponding to the maximum power value.Also, a TPC bit generator 30 compares the target power value having beenset in advance by a non-illustrated outer-loop transmission powercontrol, with the maximum power value selected by switcher 14. A TPC bit‘0’ is output when the maximum power value is greater than the targetpower value whereas a TPC bit ‘1’ is output when the maximum power valueis smaller than the target power value. Other configuration elements inboth mobile station 1 and base station 2 are similar to those shown inFIG. 1.

With this alternative embodiment, high-speed selection of a primary cellhaving good communication quality can be attained, in a similar way tothe mobile communication system shown in FIG. 1.

Additionally, in the description of the embodiments having beenillustrated, the reception characteristic of the DPCCH is measured.However, it may also be possible to measure the receptioncharacteristics (the SIR, the reception power RSCP, etc.) of otherchannels on which the transmission power control is performed in asimilar way to the DPDCH.

Further, in the foregoing description of the embodiment, W-CDMA isapplied as prerequisite. However, the present invention is not limitedto the W-CDMA system. The present invention may also be applicable forthe Code Division Multiple Access (CDMA) systems as a whole, includingthe multi-carrier CDMA. Also, it is possible to apply the presentinvention to other communication systems, such as the OrthogonalFrequency Division Multiplex (OFDM) employed as transmission techniquefor ground-wave digital broadcasting.

INDUSTRIAL APPLICABILITY

The present invention can be applied for a mobile communication system,as well as a mobile station (portable telephone, car telephone, PDA,etc.) and a base station in the mobile communication system. Forexample, the present invention is for use in a mobile communicationsystem in which communication is performed by the multi-carrier CDMAsystem including W-CDMA, and a mobile station and a base station in themobile communication system concerned.

According to the present invention, selection of the base stationtransmitting a user data signal with power control is performed based onthe quality of the control data signal transmitted with power controlsimilar to that performed against the user data signal. Therefore, asfor the user data signal, it is possible to select the base stationwhich can transmit the user data signal with the best quality, and themobile station can receive the user data of good quality.

Also, according to the present invention, in a communication conditionin which user data signal is transmitted from one base station selectedby the mobile station among a plurality of base stations, updating(switching) from the selected base station can be performed at highspeed.

Further, according to the present invention, a criterion for selectingthe base station transmitting the user data signal and a decisioncriterion of the transmission power control information are commonized,and a base station having better communication quality can be selected.

The foregoing description of the embodiments is not intended to limitthe invention to the particular details of the examples illustrated. Anysuitable modification and equivalents may be resorted to the scope ofthe invention. All features and advantages of the invention which fallwithin the scope of the invention are covered by the appended claims.

1. A mobile station having radio channels being set between the mobilestation and a plurality of base stations, and performing communicationwith the plurality of base stations in a communication condition suchthat one base station selected among the plurality of base stationstransmits a user data signal which transmission power is controlled, andthat the plurality of base stations including the selected base stationtransmit signals including a control data signal which transmissionpower is controlled in a similar way as the user data signal, saidmobile station comprising: a measurement section measuring quality ofthe control data signal transmitted with the transmission powercontrolled; a selector selecting the base station transmitting the userdata signal, based on the quality of the control data signal from eachbase station measured in the measurement section; and a transmittertransmitting identification information for identifying the base stationselected by the selector, to the plurality of base stations, wherein theuser data signal is transmitted from the base station only when theidentification information represents the base station of interest. 2.The mobile station according to claim 1, further comprising: a generatorhaving preset target quality, and comparing the target quality with thequality of the base station selected by the selector among the qualitysets measured by the measurement section, generating power controlinformation instructing to decrease the transmission power in case ofthe latter having better quality than the former, and instructing toincrease the transmission power in case of the former having betterquality than the later.
 3. The mobile station according to claim 1,wherein the measurement section measures the signal quality with asignal-to-interference power ratio.
 4. The mobile station according toclaim 1, wherein the measurement section measures the signal qualitywith a reception power.
 5. The mobile station according to claim 1,further comprising: a generator generating power control informationindicating how the transmission power of the plurality of base stationsis to be controlled, based on the quality of the base station selectedby the selector among the quality sets measured by the measurementsection, wherein the transmitter stores the identification informationand the power control information generated by the generator into eachtime slot in a frame having a plurality of time slots, and transmits theidentification information and the power control information to theplurality of base stations.
 6. The mobile station according to claim 1,wherein the plurality of base stations communicate with the mobilestation using W-CDMA, and the user data signal is a dedicated physicaldata channel signal, and the measurement section measures the quality ofa dedicated physical channel control signal.
 7. A mobile stationperforming communication with a plurality of base stations, comprising:a measurement section measuring quality of control data signals whichare transmitted from the plurality of base stations and addressed to themobile station of interest and which transmission power is controlled; aselector selecting a base station transmitting a user data signaladdressed to the mobile station of interest, based on the quality of thecontrol data signals which are addressed to the mobile station ofinterest and measured in the measurement section; and a transmittertransmitting identification information for identifying the base stationselected by the selector, to the plurality of base stations, wherein theuser data signal is transmitted from the base station only when theidentification information represents the base station of interest.